Hacker News Reader

Comment by ethbr1 | Hacker News Reader

Is the Starship upper stage boosting itself up to full orbit yet?

No. This and all previous flights have intentionlly been barely sub-orbital, with less than one orbit. Launch in Texas, re-entry over Indian Ocean. A full orbit at that altitude takes about 90 minutes; this was over in a little over an hour.

The reason was safety. If it was orbital, then controlling the re-entry would require a sucessful relight of the engines. If that failed then the re-entry point would depend upon the vagaries of orbital decay from residual atmospheric drag. That's no doubt why today's relight was so brief; they didn't want to significantly alter the reentry point.

I don't believe this is quite correct. The last few trips are actually orbital, just not of the correct elliptical shape to do more than a half orbit as the perigee is less than the radius of earth. If earth was a point mass, it would have orbited.

This means you don't have to do anything to deorbit while proving you could have made a full orbit if you wanted to.

If the earth suddenly became a point mass except for all the humans, we'd all already be in orbit. At the "top" of a highly elliptical orbit that passes relatively close to that point mass, yes, but an orbit. Everything that was within the Earth's sphere of influence, not moving so fast to be on an escape trajectory, and not with 0 horizontal velocity relative to the point mass would be in an orbit.

Orbital means "on a trajectory that doesn't intersect (or escape from) the body you are orbiting", otherwise the word is meaningless.

A person standing at the north pole wouldn't be in orbit. They would fall directly down.

(Wouldn't they?)

I concur. Right up until they exhaled and gave themselves a little horizontal velocity.

I think it would also have to be at an equinox. Otherwise the pole would be tilted towards or away from the sun, meaning that it orbits at a slightly different velocity, so you would have some velocity relative to the center of mass.

If the earth were a point mass than almost any trajectory at all would be orbital.

Can’t hit a point mass on any trajectory.

Why not? If your starting point is completely static compared to the point mass and your aspect area isn’t zero, you’re going to fall directly down towards the point mass and are going to hit it.

If it was a point mass, and you had exactly zero horizontal motion relative to it, you'd go right through and out the other side.

Well, except for relativity turning it into a black hole with a Schwarzschild radius of 8.87 mm so it won't be "point-like".

But most of the disintegrate sheen of plasma that used to be your body would have had some horizontal motion compared to it, even if only due to you starting off as an extended body.

If the point mass is inside my body for some time, I would describe that as „hitting the point mass“

If you like.

But would you say you hit neutrinos that pass through you without interaction?

If pointlike was possible, it can be similar: nothing beyond the spaghetification that happens well before you reach the event horizon.

> the perigee is less than the radius of earth

Which is what we call "sub-orbital" :)

I believe that is called a ballistic trajectory.

That's an orbital-velocity ballistic trajectory.

And that's sub-orbital. Barely

Every time you throw a rock, it ends up on a trajectory that "would have orbited if the earth was a point mass". That's just not a very useful definition of "being in orbit".

Reply on news.ycombinator.com